A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors. A varying current in the first or primary winding or first coil creates a varying magnetic field through the secondary winding or second coil. This varying magnetic field induces a varying electromotive force or “voltage” in the secondary winding.
If a load is connected to the secondary winding, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (Vs) is in proportion to the primary voltage (Vp), and is given by the ratio of the number of turns in the secondary winding (Ns) to the number of turns in the primary winding (Np) as follows:
            V      s              V      p        =            N      s              N      p      By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be “stepped up” by making Ns greater than Np, or “stepped down” by making Ns less than Np.
A transformer may also provide a galvanic isolation because charge-carrying particles that do not move from the first coil to the second coil but energy and/or information can still be exchanged between the two sections by means of an induction, electromagnetic wave, optical, acoustic, or mechanical coupling. Galvanic isolation may be used in situations where two or more electric circuits must communicate, but their grounds may be at different potentials.